In mobile communications, high user capacities and high data rates are desirable. To achieve this, mobile radio systems have to be highly spectral efficient. Using multicarrier modulation according to OFDM (orthogonal frequency division multiplexing), robust performance and high spectral efficiency can be achieved.
Before the OFDM modulation, a pre-transform can be carried out, resulting in a so-called PT-OFDM (pre-transform OFDM) system.
OFDM can be applied for MIMO (multiple input multiple output) systems. A MIMO system uses a plurality of transmit antennas (i.e. sending antennas) and receiver antennas to transmit and receive data. An example for MIMO is the VBLAST (Vertical Bell LAb's Layered Space Time) architecture described in [1]. When OFDM is combined with the VBLAST architecture, high data rate transmission can be achieved in frequency selective fading channels. It was shown in [2] that frequency diversity in a PT-OFDM system can be exploited through simple linear processing.
When the VBLAST architecture is combined with OFDM, the input data stream is modulated and assigned to different transmit antennas through serial to parallel conversion. At each transmit antenna, the data are partitioned into blocks xi of size K with element xi(m) indicating the mth signal in the block transmitted on transmit antenna i. xi is then pre-transformed using a PT (pre-transform) matrix T according toxT,i=Txi.
The pre-transformed signal is modulated according to OFDM using inverse Fast Fourier Transform (IFFT), appended with cyclic prefix (CP) and transmitted via the respective transmit antennas. The channel of the system is modeled as uncorrelated multi-path fading channel between each transmit and receive antenna pair.
A PT-MIMO-OFDM system, i.e. a MIMO system using OFDM and a pre-transform, with N transmit antennas, M receive antennas and K subcarriers can be formulated as
      R    _    =                              [                                                                                          H                    _                                                        1                    ,                    1                                                                                                                    H                    _                                                        1                    ,                    2                                                                              ⋯                                                                                  H                    _                                                        1                    ,                    N                                                                                                                                            H                    _                                                        2                    ,                    1                                                                                                                    H                    _                                                        2                    ,                    2                                                                              ⋯                                                                                  H                    _                                                        2                    ,                    N                                                                                                      ⋮                                            ⋮                                            ⋱                                            ⋮                                                                                                          H                    _                                                        M                    ,                    1                                                                                                                    H                    _                                                        M                    ,                    3                                                                              ⋯                                                                                  H                    _                                                        M                    ,                    N                                                                                ]                ⁡                  [                                                                      T                  _                                                            0                                            ⋯                                            0                                                                    0                                                              T                  _                                                            ⋯                                            0                                                                    ⋮                                            ⋮                                            ⋱                                            ⋮                                                                    0                                            0                                            ⋯                                                              T                  _                                                              ]                    ⁡              [                                                                              x                  _                                1                                                                                                          x                  _                                2                                                                        ⋮                                                                                            x                  _                                N                                                    ]              +          [                                                                  N                _                            1                                                                                          N                _                            2                                                            ⋮                                                                              N                _                            M                                          ]      where Hi,j is a K×K diagonal matrix corresponding to the single input single output frequency response of all the subcarriers between transmit antenna j and receive antenna i. T is the pre-transform matrix of size K×K and xj is the data symbol transmitted at antenna j and Ni is the AWGN noise vector at receive antenna i.
At the receiver, the received data is first transformed to frequency domain using Fast Fourier Transform (FFT) on each receive antenna. On the mth subcarrier, the received signal can be written asr(m)=H(m)xT(m)+n(m)where H(m) is the equivalent MIMO channel on subcarrier m, xT(m) is a vector formed by stacking the mth element of xT,i for i=1, . . . , M.
There are several existing detection methods for PT-MIMO-OFDM systems:
1) Linear Detection in Both Space and Frequency
In this approach, spatial domain interference suppression (IS) is first performed to separate different signals from different transmit antennas. ZF-IS (zero forcing IS) is performed asy(m)H(m)−1r(m)=xT(m)+H(m)−1n(m)where H(m)−1 indicates generalized inverse of H(m). The MMSE-IS (minimum mean square error IS) is performed asy(m)=HH(m)[H(m)HH(m)σx2+σn2IM]−1r(m)where σx2 is the variance of the transmitted signal and σn2 is the variance of the noise. IM indicates the identity matrix of size M. On each receive antenna, after MIMO processing, we have an PT-OFDM system. The decision statistic of the transmitted signal can be formed by inverse transform of yi for i=1, 2, . . . , M:di=THyi 
The advantage of this approach is its simplicity in processing. However, the problem with this approach is that neither space nor frequency diversity of the system is fully exploited. The performance is rather poor.
2) Joint Maximal Likelihood (ML) Detection Over Space and Frequency
In this approach, ML detection is carried out across both frequency and space. The advantage of the method is that the performance is good as both space and frequency is fully exploited. The complexity of this method, however, is exponential with the product of number of transmit antennas and the transform size. It is not feasible for practical implementations.
3) Spatial Domain Interference Cancellation
In this approach, linear detection is carried out, first, to obtain an estimate of the transmitted signal. Then, parallel interference cancellation and maximal ratio combining (PIC+MRC) technique is used to exploit the spatial diversity in the system.
From the decision output of the linear detector, the estimate of the transformed signal on subcarrier m, {circumflex over (x)}T(m), can be obtained. The interference cancellation for xT,i(m) can be performed as
                                                        r              _                        i            IC                    ⁡                      (            m            )                          =                                            r              _                        ⁡                          (              m              )                                -                                    ∑                                                j                  =                  1                                ,                                  j                  ≠                  i                                            M                        ⁢                                                                                h                    _                                    j                                ⁡                                  (                  m                  )                                            ⁢                                                                    x                    ^                                                        T                    ,                    i                                                  ⁡                                  (                  m                  )                                                              +                                    n              _                        ⁡                          (              m              )                                                              =                                                                              h                  _                                i                            ⁡                              (                m                )                                      ⁢                                          x                                  T                  ,                  i                                            ⁡                              (                m                )                                              +                                    ∑                                                j                  =                  1                                ,                                  j                  ≠                  i                                            M                        ⁢                                                                                h                    _                                    j                                ⁡                                  (                  m                  )                                            ⁢                                                          [                                                                    x                                          T                      ,                      i                                                        ⁡                                      (                    m                    )                                                  -                                                                            x                      ^                                                              T                      ,                      i                                                        ⁡                                      (                    m                    )                                                              ]                                +                                    n              _                        ⁡                          (              m              )                                          where hj(m) denotes the jth column of H(m). If it is assumed that the interference cancellation is perfect, i.e.
            ∑                        j          =          1                ,                  j          ≠          i                    M        ⁢                                        h            _                    j                ⁡                  (          m          )                    ⁢                          [                                    x                          T              ,              i                                ⁡                      (            m            )                          -                                            x              ^                                      T              ,              j                                ⁡                      (            m            )                              ]        =  0one has a receive diversity system with N receive antennas. Therefore, MRC can be performed on riIC(m) such thatyi(m)=hiH(m)riIC(m)
The problem with this approach is the error propagation. Decision errors in the previous iteration of detection affect the performance in the subsequent steps. For a system with smaller transform size and higher modulations (like 16 QAM), this error propagation affects the system performance significantly.
An object of the invention is to increase the performance of existing transmitting methods.
The object is solved by a method for transmitting a digital data stream, a transmitter, a method for receiving a digital data stream and a receiver with the features according to the independent claims.